Image sensor arrays typically comprise a linear array of photosensors which raster scan an image bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors.
For high-performance image sensor arrays, a preferred design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging generally without the use of reductive optics. In order to provide such a "full-width" array, however, relatively large silicon structures must be used to define the large number of photosensors. A preferred technique to create such a large array is to make the array out of several butted silicon chips. In one proposed design, an array is made of 20 silicon chips, butted end-to-end, each chip having 248 active photosensors spaced at 400 photosensors per inch.
When the individual silicon chips are butted to form a single linear array of photosensors, several practical problems must be taken into consideration. One key problem stems from the fact that each individual chip in a full-width array in many ways acts as an independent circuit, the precise outputs of which may not be necessarily consistent with the outputs of other chips forming an entire system. Specifically, no matter what particular multiplexing scheme is used to read out the various signals from each photosensor in each array, the analog video output will be dependent on the particular characteristics of each individual chip. Some chips may be of a consistently higher voltage in their outputs than others. When the signals from a plurality of chips are collected and recombined to form an image, the areas of the image which were read by particular chips, or at positions corresponding to chip boundaries, may be slightly darker or lighter than they should be because of this chip-to-chip nonuniformity.